Main components of magnetic recording apparatus such as a computer include a magnetic recording medium and a magnetic head for reproducing magnetically recorded data. As a magnetic recording medium, a floppy disc and a hard disc are known. As a substrate material for a hard disc thereof, an aluminum alloy is used. With downsizing of a hard disc drive for a notebook computer and an increase in the density of magnetic recording, recently, the flying height of a magnetic head is clearly decreasing. A magnetic disc substrate has come to be required to have very high accuracy with regard to its surface smoothness. However, even if a highly accurate polishing material and a high-precision machine tool are used for polishing an aluminum alloy, the polished surface thereof undergoes plastic deformation since the aluminum alloy has a low hardness, and it is therefore difficult to produce a smooth surface having an accuracy higher than a certain degree. Further, with the ongoing downsizing and the thickness-decreasing a substrate for a hard disc drive, it is also strongly demanded to decrease the thickness of a substrate for a magnetic disc. Since, however, an aluminum alloy has low strength and low rigidity, it is difficult to decrease the thickness of the disc with maintaining a predetermined strength which is required on the basis of the specification of the hard disc drive. A glass substrate has come on the market as a substrate for a magnetic disc required to have high strength, high rigidity, high shock resistance and high surface smoothness.
The above glass substrate for an information recording medium can be easily produced as one having a high surface smoothness (having a smaller surface roughness Rmax, Ra value) as compared with any other substrate for an information recording medium and has strength sufficient for the thickness-decreasing and downsizing, and it is gradually expanding its market share. As the above glass substrate, there are well known a chemically strengthened glass substrate whose surface is strengthened by an ion-exchange method and a crystallized substrate which is crystallization-treated.
As an ion-exchange strengthened glass substrate, there is disclosed a glass substrate, for example, a glass substrate for a magnetic disc, which is obtained by strengthening a glass containing, by % by weight, 50 to 65% of a SiO.sub.2, 0.5 to 14% of Al.sub.2 O.sub.3, 10 to 32% of R.sub.2 O (in which R is an alkali metal ion), 1 to 15% of Zno and 1.1 to 14% of B.sub.2 O.sub.3 to form a compression stress layer on the glass substrate by an ion-exchange method (JP-A-1-239036).
Further, as a chemically strengthened glass substrate, there is a substrate formed of a glass containing 60.0 to 70.0% by weight of SiO.sub.2, 0.5 to 14.0% by weight of Al.sub.2 O.sub.3, 10.0 to 32.0% by weight of an alkali metal oxide, 1.0 15.0% by weight of ZnO and 1.1 to 14.0% by weight of B.sub.2 O.sub.3 and having a linear expansion coefficient, a compression strength and a breaking strength, which are equivalent to, or greater than, specified values, respectively (JP-B-4-70262). As a crystallized glass, for example, there is disclosed a crystallized glass for a magnetic disc, which contains, by % by weight, 65 to 83% of SiO.sub.2, 8 to 13% of Li.sub.2 O, 0 to 7% of K.sub.2 O 0.5 to 5% of MgO, 0 to 5% of ZnO, 0 to 5% of PbO (in which MgO+ZnO+PbO: 0.5 to 5%), 1 to 4% of P.sub.2 O.sub.5, 0 to 7% of Al.sub.2 O.sub.3 and 0 to 2% of As.sub.2 O.sub.3 +Sb.sub.2 O.sub.3 and which contains fine Li.sub.2 O.2SiO.sub.2 crystal grains as a main crystal (U.S. Pat. No. 5,391,522).
Further, as a glass which constitutes a material for a chemically strengthened glass substrate, the following glass (a) or (b) is known.
(a) A glass comprising 55 to 62% by weight of SiO.sub.2, 10 to 18% by weight of Al.sub.2 O.sub.3, 2 to 10% by weight of ZrO.sub.2, 2 to 5% by weight of MgO, 0.1 to 3% by weight of BaO, 12 to 15% by weight of Na.sub.2 O, 2 to 5% by weight of K.sub.2 O, 0 to 7% by weight of P.sub.2 O.sub.3 and 0.5 to 5% by weight of TiO.sub.2, the total amount of Al.sub.2 O.sub.3 and TiO.sub.2 being 13 to 20% by weight (see JP-A-1-167245).
(b) A glass comprising 64 to 70% by weight of SiO.sub.2, 14 to 20% by weight of Al.sub.2 O.sub.3, 4 to 6% by weight of Li.sub.2 O, 7 to 10% by weight of Na.sub.2 O, 0 to 4% by weight of MgO and 0 to 1.5% by weight of ZrO.sub.2 (see JP-B-6-76224).
Meanwhile, with the recent downsizing and thickness-decreasing of a substrate for a hard disc drive and an increase in recording density, the flying height of a magnetic head rapidly decreases and the disc rotation rapidly increases, so that a material for a disc substrate is required to satisfy more severe strength, Young's modulus, surface smoothness, and the like. Particularly, with an increase in the recording density of a 3.5 inch hard disc for a server and with an increase in the substrate rotation speed which is required with an increase in the speed of data processing, the requirement of rigidity of a substrate material has been more severe, and the conventional aluminum substrate is reaching its limit. So long as demands of the higher capacity, the downsizing and the higher rotation speed of a hard disc are inevitable in the future, a substrate material for a magnetic recording medium will be strongly required to comply with a decrease in thickness, higher strength, a higher rigidity degree, higher smoothness and higher impact resistance.
However, chemically strengthened glass substrates as disclosed in the above JP-A-1-239036, etc., have a Young's modulus of approximately 80 GPa, and it may not be possible to satisfy severe demands required of a substrate for a hard disc drive in the future. There has been found the following defect; a conventional glass which is chemically strengthened by ion exchange contains a large amount of an alkali component, and when it is used in a high-temperature and high-humidity environment, alkali ion diffuses from a pinhole portion of the magnetic film, a portion where a magnetic film has a small thickness such as a vicinity of the magnetic film or a portion where glass is exposed, which triggers the corrosion or alteration of the magnetic film.
The conventional crystallized glass as disclosed in U.S. Pat. No. 5,391,522 has somewhat excellent Young's modulus and heat resistance over the above chemically strengthened glass. However, the problem is that it is inferior to glass in surface roughness and has its limit concerning a decrease in the flying height of a magnetic head, so that it cannot comply with an increase in the density of magnetic recording.
Further, when the rotation speed of a magnetic disc formed of a conventional glass substrate for an information recording medium is increased for speeding up the date processing, it is difficult to secure a stable flying height (distance between a magnetic head and a magnetic disc during the reproduction of recorded data).
A magnetic recording medium utilizing a glassy carbon as disclosed in JP-A-3-273525 is excellent over the above chemically strengthened glass and crystallized glass in heat resistance and light weight. However, it has may surface defects and it is presumably not sufficient for high-density recording. Further, since the glassy carbon has a very low Young's modulus and low strength, it is required to increase the thickness of the substrate, which results in a problem that it cannot comply with a decrease in the thickness of a substrate.